This invention relates to a light-transmitting shield for protecting ultraviolet-emitting lamps from casual impact and for retaining lamp fragments and debris should the frangible envelope lamp be broken.
The invention is particularly concerned with providing a protective shield for a fluorescent lamp having an elongated tubular glass envelope containing a low pressure ionizable medium. Breakage of the glass envelope of such a lamp can result in implosion with resulting violent scattering of fragments of glass and fluorescent powders, unless contained by a surrounding shield. Typically such shields for fluorescent lamps have comprised tubular sleeves of a polycarbonate, a UV absorbing acrylic or a styrene plastic, as described in U.S. Pat. No. 3,124,307 Hoskins et al, U.S. Pat. No. 3,673,401 DuPont, U.S. Pat. No. 3,720,826 Gilmore et al, U.S. Pat. No. 3,798,481 Pollara, and U.S. Pat. No. 3,808,495 Ulin. Although such prior art plastic materials may be quite satisfactory for conventional general lighting applications of fluorescent lamps, these materials have been found quite unsatisfactory for applications wherein the lamps are designed to emit a significant amount of ultraviolet radiation during normal operation.
For example, a copending application Ser. No. 693,029, filed concurrently herewith and assigned to the present assignee, describes a photochemotherapy chamber containing a plurality of special fluorescent lamps adapted for emitting long-wave ultraviolet light (UVA) in the region of 320 to 400 nanometers. Exposure to such radiation subsequent to oral administration of psoralens has been observed to artificially induce natural tanning of the skin of the human body. In view of clinical studies in this area, such therapy appears to have significant dermatological application with respect to the medical treatment of various skin disorders.
A critical component of the photochemotherapy procedure is, of course, the irradiation apparatus; it must safely provide the proper light radiation in an efficient yet carefully controlled manner. If extensive proportions of a person's body are to be irradiated and the individual is not bedridden, an upright enclosed chamber containing an array of lamps disposed to substantially surround a standing person with light is particularly useful in this application. For the protection of a person standing inside the chamber, each of the fluorescent tubes is enclosed in a protective plastic sleeve. Perferably there is a spacing between the glass tubing of the lamp and the enclosing sleeve, and the sleeve is retained by plastic end caps. In this manner, the lamps are protected from casual impact, and should the lamp be broken, the lamp fragments will be retained by the sleeve and end cap assembly. Of course, the plastic material of which the sleeve is formed should be of a type which efficiently transmits the UVA light and remains stable (i.e., will not discolor and disintegrate) under continued exposure thereto.
As previously mentioned, the prior art protective sleeve materials contend only with the heat and ultraviolet radiation from a standard fluorescent lamp used for general lighting applications. When these prior art protective sleeve materials were tested for use on lamps of the type intended for the above-described photochemotherapy chamber, however, it was found that the ultraviolet transmittance deteriorated rapidly under the UVA radiation. A typical example would be where a sleeve transmits 80% of the required radiation at the beginning of lamp life, but is reduced to about 40% after 400 hours operation. Usually the material yellows and begins to deteriorate physically at that point.
FIG. 1 shows a specific example of the spectral transmittance curves measured for a section of material taken from a prior art commercial lamp sleeve formed of polycarbonate. A Cary Spectral Photometer was used. One curve represents the initial transmittance, while the second curve shows the transmittance after 900 hours of exposure to 8 milliwatts per square centimeter of radiant energy in the ultraviolet wavelength range of between about 300 and 400 nanometers. It will be noted that the initial transmittance below 330 nanometers is poor, and with continued exposure, the transmittance decreases radically throughout the denoted ultraviolet range.
The lighting industry often uses plastics that are "UV stabilized" to prevent yellowing upon exposure to ultraviolet radiation. However, these stabilizers are UV blocking agents, and such stabilized plastics do not transmit appreciable ultraviolet radiation.